"Nature Abhors a Vaccuum": Invagination of the Small Intestine into the Lumbar Disc Space After a Spinal Fusion Operation

A 77-year-old woman having back pain due to an L2 vertebral body compression fracture took a lumbar spine magnetic resonance imaging (MRI). In MRI, in addition to the L2 vertebral body fracture, invagination of the small intestine into the intervertebral disc space at L5-S1 was found by chance. On a lateral lumbar spinal X-ray, the lordotic angle was markedly increased at the L5-S1 level. Additionally, the L5-S1 disc space had widened. These X-ray findings indicate the segmental instability at L5-S1. The spinal fusion operation on L3-4-5 seems to have resulted in overt mechanical loading on the inferior spinal segment (L5-S1). We think the instability damaged the anterior longitudinal ligament and caused a tear in the anterior portion of the annulus fibrosus. The defect in the L5-S1 intervertebral disc after the tear would have caused the vacuum, which is presumed to have pulled the patient's small intestine into the empty space within the L5-S1 intervertebral disc. Although intervertebral invagination of intra-abdominal structures is not common, clinicians should be aware of the possibility of this complication in patients who have spinal segmental instability.